1. Field of the Invention
The present invention relates to a shift control apparatus for an automatic transmission. More particularly, the present invention relates to a shift control apparatus which performs a downshift when an accelerator is off (i.e., during power-off), more specifically, which controls engine output during a downshift.
2. Description of the Related Art
Typically in a vehicle having an automatic transmission, when sufficient engine braking force is unable to be obtained when the accelerator is off (i.e., power-off), when traveling down a slope or the like, a driver routinely downshifts by either shifting a shift lever from the D-range to the S-range or 1st-range, or by operating a manual shift device to obtain the desired engine braking force. Also, automatic transmissions, recently designed to keep the lock-up clutch applied to the extent possible, downshift in response to deceleration when coasting (i.e., power-off) at a predetermined speed and keep the engine in a driven state (in which the rotation of the wheels drive the engine, instead of the engine driving the wheels) at idling speed or faster for a long time in an attempt to improve fuel efficiency.
During such a power-off downshift there is a time lag between start of the transmission of torque on the output side to the engine by application of a frictional engagement device (i.e., a clutch or brake) for the shift and the point in time when the engine speed increases, relative to the input shaft speed. Thus, as the shift duration increases so does the friction energy expended by the frictional engagement device, causing its friction elements to wear faster. Conventionally, to eliminate this problem, a shift control apparatus shortens the shift duration while suppressing shift shock by increasing engine output when a predetermined timed period has passed after start of timing in a power-off downshift (see Japanese Patent Application Laid Open (“Kokai”) No. 5-338469). The shift control apparatus disclosed in this publication changes the duration of the timed predetermined period in accordance with changes in the hydraulic temperature and the engine speed in order to prevent, for example, conveyance of an adverse sensation to the driver due to an increase in drive torque or the like. It also assures sufficient reduction in shift duration, notwithstanding any offset of the timing of engine racing by a delay in the apply or release of the frictional engagement devices or a delay in the change in engine output.
However, even if engine output is appropriately increased with accurate timing in a predetermined downshift of, for example, 3rd speed to 2nd speed, by changing the duration of the timed predetermined period, it is still difficult to optimally adapt such control to all downshifts. In particular, in a skip shift from 4th speed to 2nd speed, for example, the offset in the timing of engine racing is due more to the type of shift than it is due to external factors such as hydraulic temperature, in which case the engine output increase control may not be initiated at the appropriate time. In particular, in an automatic transmission to which a manual operating device has been added, power-off downshifts are frequently performed by manual operation. At such time, a variety of types of shifts such as normal shifts and skip shifts are possible. Therefore, engine control with inappropriate timing and with an inappropriate amount of torque increase may place the engine in a driving (i.e., power-on) state, which may convey an uncomfortable sensation to the driver or generate shift shock.